In drilling and boring holes in earth formations, especially those formations that are of relatively hard nature, it is typical for rotary diamond bits to be employed. Diamond bits are rotated in the presence of a circulating drilling fluid medium that serves to achieve cooling of the bit during its cutting operation and to remove drill cuttings from the cutting face and sides of the bit as drilling operations progress. In most diamond bits, the diamonds project from the bit matrix or body in which they are embedded with projection being such that the diamonds contact the formation in such manner as to provide a clearance between the matrix and the surface of the formation. The lands defined by the matrix generally define fluid courses that radiate outwardly from one or more apertures through which drilling fluid medium is ejected through the bit structure into the well bore. Cooling of the bit and particulate removal occur as the drilling fluid medium flows across the land at high velocity in the space between the matrix and the formation. Penetration of the bit into the formation is generally controlled by the mechanical pressure that is applied to the bit as drilling operations continue. Of course, the formations vary considerably and therefore penetration of the diamonds into the formation tends to vary considerably. As is expected, variation in the penetration of the diamonds results in substantial change in the effective size of the clearance between the matrix and the formation and consequently causes the cooling ability of the fluid supply system to be varied.
At times drill cuttings will become trapped between the bit and the formation and the bit, thus developing extremely high compression loads on the bit which can result in excessive wear. Also, quick and repetitive accumulation and release of drill cuttings from beneath the bit can cause chattering and vibration that is also detrimental to the life of the bit.
A major problem with diamond bits concerns the degree of diamond wear that occurs during drilling operations. Because the diamonds are much harder than the formation materials that are drilled, what is typically taken as "wear" generally concerns the amount diamond fracturing that occurs as diamond drill bits are used. Where several diamonds are positioned in the matrix such that drilling fluid transports particulate from a leading diamond to a trailing diamond in some cases the particulate becomes wedged between the trailing diamonds and the formation. Suddenly, the trailing diamond is subjected to a tremendous compression load due to the wedging action and this load causes the diamond to fracture. After each of the diamonds of a bit structure have become fractured, the diamonds are considered to have become worn to the point that its cutting ability is not proper. It is deemed that extended cutting life of diamond drill bits will be materially enhanced if the drill cuttings are not allowed to engage trailing diamonds and develop shearing loads. It is also considered that excessive fracturing of diamonds will not occur if the particulate material is immediately projected into the waterways and transported away from the cutting side.
Another problem associated with diamond drilling bits concerns the inability of the penetration rate of the bit to be precisely controlled as drilling operations continue. As mentioned above, penetration of the diamonds into the formation is generally determined by the hardness of the formation and the amount of mechanical force applied to the drill bit as it is rotated relative to the formation. It is considered desirable to limit the amount of penetration that can occur and to insure that each of the diamonds on the cutting face of the bit penetrate into the formation a predetermined amount and maintain such predetermined penetration at all times. Drill bits tend to chatter and vibrate against the formation because penetration of the diamonds cannot be precisely controlled. This chattering and vibration is extremely detrimental to the service life of diamond drill bits.
A further problem concerns the typical inability of diamond drill bits to accomplish efficient drilling near the center of the contact area between the face of the bit and the formation. The outer portions of diamond bits typically drill more rapidly than the inner or apex portions primarily because of inefficient cleaning capacity of the flowing drilling fluid and the multiple stone center without clearance for cutting. The fluid supply and control arrangement afforded by most diamond drill bits causes fluid flow to be rather sluggish at the central portion of such bits, causing transportability of the drill cuttings to be rather poor. The cuttings interfere with the cutting action of the innermost diamonds and cause increased wear and slow drilling at the apex center. This causes the bit to hang on its center and vibrate or chatter in the hole.
As mentioned above, it is desirable that drill cuttings be removed from the cutting face of the bit as soon as possible after separation from the formation. It is desirable to insure high velocity flow of the drilling fluid medium through the fluid courses in order that the drilling cuttings loosened by the diamonds are efficiently transported from the drilling site. It is desirable to provide drill bit apparatus that is designed to utilize the rotary speed of the drill bit to materially enhance the cleaning capacity of the flowing drilling fluidmedium.